FRANKFURT, April 10 (Reuters) - One of Bayer's largest shareholders tore into the company's management on Wednesday for underestimating the legal risks of its takeover of Monsanto, setting the stage for a fiery annual general meeting after a 30 percent plunge in the shares.

Bayer has seen about 30 billion euros ($34 billion) wiped off its market value since August, when a U.S. jury found Bayer liable because Monsanto had not warned of weedkiller Roundup's alleged cancer risks. It suffered a similar courtroom defeat last month.

"It's quite drastic when a takeover triggers such value destruction and reputational damage so quickly. There can be no talk of a successful takeover any more," Ingo Speich, the head of sustainability and corporate governance at Deka Investment, told Reuters. He will be among the shareholders to speak at the April 26 annual general meeting (AGM).

"What's startling is that things have effectively moved beyond management's control because we're now at a point where the decisions over future development are made in court rooms," he said, adding Bayer had clearly underestimated the risks.

With a holding of about 1 percent, Deka says it is Bayer's 10th-largest shareholder overall, and the second-largest German one after DWS.

Major shareholder advisory firms Institutional Shareholder Services (ISS) and Glass Lewis last week recommended investors not to give the executive board their seal of approval at the AGM. Glass Lewis also advised against a vote of approval for the non-executive supervisory board.

A shareholder vote to ratify boards' actions features prominently at every German AGM. Though largely symbolic, as it has no bearing on management liability, it is seen as a key gauge of shareholder sentiment.

Among shareholders that have vowed to deny top Bayer executives their vote of confidence is the former head of DWS, Christian Strenger.

Deka's Speich declined to disclose how Deka would vote.

Company filings showed this week that Bayer's supervisory board sought law firm Linklaters' expert opinion for reassurance that the management board had complied with its duties when acquiring Monsanto for $63 billion last year.

The supervisory board in Germany's two-tier corporate board system has to sign off on larger transactions. Bayer's non-executive Chairman Werner Wenning was a driving force of the Monsanto deal, according to sources familiar with the matter.

Bayer is appealing, or has vowed to appeal, the two jury verdicts, but more than 11,000 plaintiffs are seeking damages.

The U.S. Environmental Protection Agency, the European Chemicals Agency and other regulators across the globe have found that glyphosate, the active ingredient in Roundup, is not likely carcinogenic to humans.

However, the World Health Organization's cancer arm in 2015 reached a different conclusion, classifying glyphosate as "probably carcinogenic to humans."

Speich said Bayer could now be vulnerable to attacks from activist shareholders who might seek to break it up, although the "bitter pill" of litigation risks might deter them for now.

Such risks could also distract management, he added, when it "should be fully absorbed by the integration of Monsanto," Speich said.